As a method for bonding objects to be bonded each having a bonding portion, Japanese Patent 2,791,429 discloses a bonding method of silicon wafers for, at the time of bonding the bonding surfaces of the silicon wafers to each other, sputter etching the bonding surfaces by irradiating an inert gas ion beam or an inert gas high-speed atomic beam to the surfaces at a vacuum condition with a room temperature prior to the bonding. In this bonding method, oxides or organic substances on the bonding surfaces of the silicon wafers are removed by the above-described beam and the surfaces are formed by atoms activated by the beam, and both surfaces are bonded to each other by a strong bonding force between the activated atoms. Therefore, in this method, basically heating for bonding is not necessary, and it is possible to bond the objects at a room temperature or a low temperature close to the room temperature merely by bringing the activated surfaces into contact with each other.
In this bonding method, however, the bonding of the etched bonding surfaces must be carried out in a vacuum at a condition where the surface activated state is maintained. Therefore, the predetermined vacuum condition must be maintained from the surface cleaning by the above-described beam to the bonding, and in particular, because at least a part of the mechanism for bonding must be constructed in a chamber capable of maintaining the predetermined vacuum degree, a large-scale sealing mechanism is required, and the whole of the device becomes large and expensive. Further, if it is tried to carry out the surface cleaning and the bonding at different places for separating the surface cleaning process by the above-described beam and the bonding process from each other, it is required to maintain a predetermined vacuum condition between both places and mean for conveying the objects from a cleaning place to a bonding place while maintaining the predetermined vacuum condition is required, and therefore, it becomes difficult to design a practical device and the whole of the device becomes large.